JOURNAL OF CATALYSIS 75, 233-242(1982) Benzene Hydrogenation over Nickel Catalysts at Low and High Temperatures: Structure-Sensitivity and Copper Alloying Effects G.A. MARTIN AND J.A. DALMON Institut de Recherches SW lu Catalyse (CNRS), 2 Atvenue Albert Einstein, 69626 Villeurbanne CPdrx, Frrrr~ce Received February 9, 1981 The rate of benzene hydrogenation has been measured over various silica-supported Ni, unsup- ported Ni, and Ni-Cu/SiOz catalysts from 300 to 600 K, and has been referred to unit area of metal. It is shown that benzene hydrogenation at low temperature is to be considered as a structure- sensitive reaction. As particle size increases from 2.5 to 6 nm, the activity increases. Reduction of catalysts at high temperature results in a marked decrease of activity. This behavior is very similar to that observed for ethane or propane hydrogenolysis. Copper addition slowly decreases the activity. The active site is an ensemble composed of 3 7 2 adjacent nickel atoms. The influence of particle size and of copper alloying on the rates of benzene conversion into cyclohexane or methane at high temperatures is quite different from that observed at room temperature. Finally, a correla- tion between alloying effects and particle size-sensitivity for particles smaller than 6 nm is pre- sented. INTRODUCTION metallic atoms involved in the active site (6). In this respect, of special interest is the Although benzene hydrogenation into cy- influence of alloying nickel with copper on clohexane has received much attention (I), the rate of benzene hydrogenation. Some no consensus of opinion exists in the litera- data are already available in the literature ture about the structure-sensitivity of the (7, 8). The activity of nickel toward ben- reaction. The rate of reaction per unit me- zene hydrogenation at low temperatures tallic area over nickel catalysts is generally (below 373 K) decreases slowly as copper considered as being little influenced by sur- concentration increases (7, 8), while it in- face structure (2, 3). There are, however, creases at high temperatures (8). In both some papers dealing with benzene hydroge- cases, however, the reaction was carried nation over nickel catalysts which report a out on films (7) or powders (8) which are more or less marked structure-sensitivity. known to exhibit a strong surface enrich- On the one hand, Coenen et al. (4) found ment in copper, thus making difficult a small nickel particles (size smaller than 1.2 quantitative study of the alloying effect. nm) to be less active than large particles by This situation has prompted us to reex- a factor of 5. On the other hand, Selwood et amine the problem of benzene hydrogena- al. (5) reported a drop in activity which tion on nickel, and this paper presents new greatly exceeds the fall of specific surface data on the structure-sensitivity of the reac- area when nickel catalysts are thermally tion and on the influence of copper addition sintered. This shows that the options on on the activity of well-defined catalysts. structure-sensitivity of benzene hydrogena- tion are far from being completely unequiv- EXPERIMENTAL ocal. The morphological characteristics of the It has been suggested that the structure- catalysts, which were partially described in sensitivity of catalytic reactions could be previous papers (9, II), are summarized in related to alloying effects and that both phe- Table 1. Unsupported nickel powder sam- nomena could be ascribed to the number of ples were obtained by decomposition of 233 0021-95 17/82/060233-10$02.00/O Copyright 0 1982 by Academic Press. Inc. AU rights of reproducflon in any form reserved. 234 MARTIN AND DALMON TABLE 1 Sample Morphology of Nickel Catalysts Number Precursor Ni Reducing Degree of DS S loading temperature reduction @ml (m”/g Ni) 1% wt) f”K) 1 Ni(OHh 62 520 80” 12 2 Ni(OHh 62 570 1.01 67” 10 3 Ni(OH)* 62 670 96” 7 4 Ni(OH), 62 770 167O 4 5 Ni(OH)&SiOz (hexamine) 4.5 800 0.95 2.5 6 Ni(OH)JSiOZ (hexamine) 4.5 920 1 3.2 7 Ni(OH)JSiOz (hexamine) 11 970 1 5.7 8 Ni(OH).JSiO, (hexamine) 23 920 0.98 6.3 9 Ni(OH),/SiO, (hexamine) 23 lOoa 1 7.5 10 Ni(OH)JSiOz (hexamine) 23 1200 1 14 11 Ni*+/SiO, (ex nitrate) 6 770 1 12 a As deduced from the BET surface area assuming spheres. nickel hydroxide (prepared by the ammonia with diameters of the other samples calcu- method) in flowing helium for 1 hr, followed lated from magnetic data. The Ni-Cu/SiOz by reduction for 2 hr in flowing hydrogen catalysts were prepared by reduction at 920 (gas flow, at a rate of 4 liters/hr), at temper- K in flowing hydrogen of precursors ob- atures listed in the table. Surface areas were tained by adding silica to solutions of nickel calculated from nitrogen adsorption data at and copper nitrate hexamine (12). Ad- liquid nitrogen temperature. The Ni/SiOz sorption and magnetic studies have shown catalysts were prepared by reduction for 15 that a homogeneous alloy was formed and hr of precursors obtained by reacting the that the surface composition of the metallic support (SiOs, Aerosil Degussa, 200 m*/g) particles (diameter, 6 nm) was very similar with a solution of nickel nitrate hexamine to the bulk composition (12), in contrast (samples 5 to 10) or with a solution of nickel with the surface enrichment in copper gen- nitrate (sample 11). Reduction temperatures erally observed on Ni-Cu films and unsup- and metallic loadings were varied to cover a ported powders. wide range of particle sizes. Prior to reduc- Kinetic experiments were carried out in a tion, samples 5 and 11 were treated in va- flow system with a fixed-bed reactor at at- cuo for 1 hr at the reduction temperature. mospheric pressure. The microreactor con- Magnetic measurements showed that re- sisted of a quartz tube with a porous disk duction of the samples was almost complete and samples were held by quartz wool. The and also allowed us to calculate the surface total flow at room temperature was 120 ml/ average diameters, D, listed in Table 1 min. Hydrogen and helium (used as a dil- (9, II). The volumes of adsorbed hydrogen uent) had an initial purity better than at saturation were found to be in agreement 99.99%, and were further purified by a De- with the BET surface area of sample 2, and 0x0 catalyst followed by a zeolite trap. Par- BENZENE HYDROGENATION OVER Ni 235 tial pressures of benzene were varied by crease of activity with time obeys a simple bubbling a flow of helium in liquid benzene. logarithmic law which allows extrapolation Pure thiophene-free benzene from Merck to initial time. The variations of the initial was further purified by adding metallic so- activity per unit area with metallic particle dium and by a clean Pt/SiO, catalyst to diameters are shown in Fig. 2. Experimen- remove minute amounts of sulfur com- tal points have been arranged more or less pounds (no important difference of behav- arbitrarily into two groups (curve A and B ior however, was detected between initial in Fig. 2) corresponding to data obtained and purified benzene). Gas analyses were with samples reduced at temperatures performed by gas chromatography with a higher and lower than 770 K, respectively. flame-ionization detector. Conversion was Results obtained by Coenen et al. (13) and always less than 2% except for aging exper- by Nikolajenko et al. (2) can be compared iments. The rates reported here refer to the to ours in Fig. 3. Figure 4 shows the activi- rate of conversion of the parent hydrocar- ties of samples 5-10 toward various reac- bon (benzene). tions, namely, benzene hydrogenation, and hydrogenolysis of ethane and propane (14, RESULTS 1.5). From these data, it can be seen that the Benzene Hydrogenation at Low* various activities vary more or less in a par- Temperatures allel way, particularly for particle diameters The variations of catalytic activity with larger than 6 nm. Below this critical diame- time were first examined. As can be seen in ter, the activities decrease with nickel parti- Fig. 1, the phenomenon of catalyst aging is cle size and the corresponding slopes vary observed for samples prepared below 520 according to the reaction considered. Varia- and 770 K for unsupported and supported tions of the rate of benzene hydrogenation catalysts, respectively. The observed de- at low temperature with the copper concen- trationx are shown in Fig. 5, from which it can be deduced that the variations of the LOG conversion activity A, with x can be approximately represented by -1 A, = A,(1 - x)” with N = 3 T 2. Apparent activation energies, E,, and partial reaction orders with respect to hy- -2 drogen and benzene partial pressure, nH and c.m3Tr~- 3 nB, of the samples were also measured. The results thus obtained can be compared in Table 2. To a first approximation, they can be considered as constant, showing that the -3 mechanism does not change with particle size and alloying. O\ 1 Reaction of Benzene with Hydrogen at TIME]min ) High Temperatures 30 60 The reaction was also studied over a wide FIG. 1. Catalyst aging: Log conversion as a function range of temperature. A typical Arrhenius of time for samples 1, 11, 3, and 8 (curves 1-4, respec- plot showing the influence of temperature tively). on conversion is represented in Fig. 6. As 236 MARTIN AND DALMON LOG r (molecules/sea/cm2 Ni ) 13 12 11 LOG Ds(nm) 1 1 .5 1 1.5 2 FIG.